In Illinois, the influence of Chicago (Cook County) is overwhelming. This county contains over 40% of IL's total population, and every election over the past three decades have results in Democratic blowouts at the presidential level. In addition, the suburban collar counties surrounding Cook have voted Democratic over the past 10 years.

Downstate, there are some Democratic bastions, primarily in counties with major universities (Champaign, Jackson), as well as some counties with relatively high populations. However, Trump did manage to flip a string of counties in NW Illinois, as this region is located next to NE Iowa & SW Wisconsin (both of which flipped to Trump). Nonetheless, this shift was offset by Democratic gains in the Chicago area and other Democratic bastions mentioned above.

Because Chicago and its suburbs swung D, and that's the vast majority of the state population. Add McLean and Champlain giving solid swings towards Hillary too, and it doesn't matter that the small and declining rural counties downstate swung hard to Trump.